The main elements for LCD (Liquid Crystal Display) are polarizer and backlight module. The backlight module produces uniform light, and then, the light is polarized by polarizers with different colors so as to generate multi-color picture. For generating uniform light, the backlight module has to equip with multiple long life lamp tubes and an inverter for providing power thereto, wherein the brightness of the lamp tube can be altered through adjusting the magnitude of the output power from the inverter. The conventional inverter, as shown in FIG. 1, includes a power source 1, a dimming signal source 6, a PWM (Pulse Width Modulation) unit 3, a switch unit 2, a resonance unit 4, a transformer 5 and a feedback unit 7. The power source 1 provides DC power, the PWM unit 3 produces a working cycle signal to drive the switch unit 2, and the DC power is switched by the switch unit 2 to form a pulse power. The resonance unit 4 obtains the pulse power and converts thereof into a driving power through resonance, and then, the transformer 5 converts the driving power into an output power to drive a lamp tube set 9. The dimming signal source 6 produces a dimming signal to the PWM unit 3 for adjusting the duty cycle of the working cycle signal so as to control the magnitude of the output power. The feedback unit 7 draws a feedback signal from the secondary side of the transformer 5 and transmits the feedback signal to the PWM unit 3 for achieving feedback regulation. In the conventional inverter, the duty cycle of the working cycle signal is utilized to control the magnitude of the output power. Mostly, the resonance circuit used to drive the inverter of the backlight module is an LC resonance circuit. The equivalent circuit of basic LC resonance circuit is shown in FIG. 2A, wherein Rlamp represents equivalent resistor of a lamp tube, Vd(t) represents input voltage, Ls represents resonance inductor, Cp represents resonance capacitor and the input voltage is a pulse power of constant voltage, and Vλ represents potential difference of lamp. The input voltage and the output voltage are calculated as followed:
                                                        ν              λ                        ⁡                          (              jω              )                                                          ν              d                        ⁡                          (              jω              )                                      =                                            R              lamp                        //                          1                              jω                ⁢                                                                  ⁢                                  C                  p                                                                                        jω              ⁢                                                          ⁢                              L                s                                      +                          (                                                R                  lamp                                //                                  1                                      jω                    ⁢                                                                                  ⁢                                          C                      p                                                                                  )                                                          (                  1          ⁢                      -                    ⁢          1                )            
After transposition, it obtains
                                                                                                              v                    λ                                    ⁡                                      (                    jω                    )                                                                                        v                    d                                    ⁡                                      (                    jω                    )                                                              =                                                                    R                    lamp                                                        1                    +                                          jω                      ⁢                                                                                          ⁢                                              C                        p                                                                                                                                  jω                    ⁢                                                                                  ⁢                                          L                      s                                                        +                                                            R                      lamp                                                              1                      +                                              jω                        ⁢                                                                                                  ⁢                                                  C                          p                                                                                                                                                                                            =                                                R                  lamp                                                                                            (                                              jω                        ⁢                                                                                                  ⁢                                                  L                          s                                                ⁢                                                  R                          lamp                                                                    )                                        ⁢                                          (                                              jω                        ⁢                                                                                                  ⁢                                                  C                          p                                                                    )                                                        +                                      jω                    ⁢                                                                                  ⁢                                          L                      s                                                        +                                      R                    lamp                                                                                                          (                  1          ⁢                      -                    ⁢          2                )            
For simplifying the formula described above, it further defines that:
                    Q        =                              R            lamp                                                              L                s                                            C                p                                                                        (                  1          ⁢                      -                    ⁢          3                )            
wherein Q is defined as serial resonance quality factor, and
                              Z          o                =                                            ω              o                        ⁢                          L              s                                =                                    1                                                ω                  o                                ⁢                                  C                  p                                                      =                                                            L                  s                                                  C                  p                                                                                        (                  1          ⁢                      -                    ⁢          4                )            
wherein ZO is the property impedance of the resonance circuit.
Through formulas (1-3) and (1-4), formula (1-2) can be simplified as followed:
                                                        v              λ                        ⁡                          (              jω              )                                                          v              d                        ⁡                          (              jω              )                                      =                  1                      1            -                                          (                                  ω                                      ω                    o                                                  )                            2                        +                          j              ⁢                              1                Q                            ⁢                              (                                  ω                                      ω                    o                                                  )                                                                        (                  1          ⁢                      -                    ⁢          5                )            
Please refer to FIG. 2B, which shows the gain curve of transfer function of LC resonance circuit. The conventional LC resonance circuit is operated under fixed frequency so as to form fixed voltage gain. Through the PWM unit 3, the duty cycle of the working cycle signal can be altered for adjusting the brightness of the lamp tube set 9, as disclosed in R.O.C. Patent No. 1290707, entitled “Parallel driving circuit of multiple lamps in LCD and a uniformity control method thereof”. In FIG. 3 of this patent, a resonance inductor (Lr) and a resonance capacitor (Cr) are disposed at the primary side of transformer T1. Through the sine wave produced by the resonance inductor (Lr) and the resonance capacitor (Cr), the transformer T1 can convert the power. However, the inverter described above which controls the brightness through altering the working cycle signal is disadvantageous that:
1. The adjustment of the duty cycle of the working cycle signal will cause the switch unit of the inverter to have an unstable zero voltage switching, so as to cause extra loss.
2. The adjusting range of the duty cycle is limited since the elements of the switch unit have limited voltage withstand.
Therefore, there is the need to improve the inverter used for driving backlight module.